It is commonly estimated that about 80% of Americans will see a health care professional at some point in their lives for back problems. Many people claim ongoing back pain as a permanent malady. The majority of these problems are related to the spinal discs. For example, about 250,000 Americans each year undergo surgery for herniated discs.
The spinal column is comprised of 26 interlocking vertebrae. These vertebrae are separated by discs. The spine serves two basic functions: (1) it provides load bearing support for one half of the body's mass and (2) it protects the nerves of the spinal column. The discs serve to provide shock absorption as well as to facilitate the bending of the spine. The discs are formed of a cartilaginous outer structure, the annulus fibrosus, with a gel-like inner structure, and the nucleus pulposus. The nucleus pulposus begins with approximately a 80% water content, but gradually during a person's lifetime the nucleus pulposus loses water and its ability to absorb shock. The disc is susceptible to numerous ailments including, but not limited to, degeneration, bulging, herniation, and thinning, as well as vertebrae related ailments such as osteophyte formation.
Treatment of these ailments has been limited to only a few courses of action. Fusion, or spinal arthrodesis, is a commonly used technique in spine surgery. Clinical studies increasingly suggest that while this surgical technique is likely to result in short-term relief, it does not effectively address the progressive degeneration of the patient's condition in the long run. Fusion restricts the range of motion available to the patient. Further, spinal arthrodesis is associated with premature degeneration of adjacent motion segments, due to the transference of additional stresses and motion from the immobilized segment to the adjacent unfused levels.
Sir John Charnley revolutionized modem orthopedics with the introduction of total hip replacement. Hip and knee arthroplasties are currently two of the most highly rated surgical procedures in terms of patient satisfaction. However, a correspondingly successful and safe method and device for spinal arthroplasties has not been developed.
Artificial disc replacements attempt to accomplish this. The objective of artificial disc replacement is to restore the normal disc height while preserving the natural movement between the vertebrae, thereby alleviating the pain caused by degenerative disc disease. There are two main categories of disc replacement devices: total artificial discs and prosthetic disc nucleus replacements. With respect to the latter, which replace only the soft center of the disc while retaining the firmer disc walls, disc spacer devices can restore normal height, but provide no assurance that a proper seal or structural integrity are achieved.
The average age of a disc replacement patient is under 40, meaning that a disc prosthesis must frequently last 40 or more years. This long product lifetime and the fast pace with which improvements in synthetic discs are being made has created a need for a prosthesis that will provide the surgeon with the ability to easily revise, update, upgrade or replace the disc replacement postoperatively. Additionally, there is a need for surgeons to intraoperatively adjust or modify the prosthesis to compensate for surgical factors. Further, there is a need for an artificial disk prosthesis system that leaves the end plate of the vertebrae adequately exposed such that arthrodesis could be performed postoperatively without removal of the entire prosthesis device. An artificial disc is also needed that can be utilized as a part of a modular artificial disc prosthesis and that provides a degree of shock absorption and flexibility similar to that of a natural disc.